More and more operations in the manufacture and packaging of goods require a sensitive means of detecting and removing out-of-specification, cans, containers, cartons, packages and the like. The items to be removed may not be within certain weight limits, size, position, or shape to pass a particular test. With the rapidity that the modern machines have to run to satisfy production requirements, the usual visual methods of screening is not satisfactory.
In the past, vacuum devices have been used to automatically detect and reject downed cans. With devices of this type, a vacuum is applied to the open upper end of upright cans to hold the cans against a moving conveyor, while the downed cans will not be held against a vacuum mechanism and are rejected to a collection station.
One such prior art device is disclosed in U.S. Pat. No. 4,146,467 to Sauer, et al. wherein a pair of endless belt conveyors are spaced apart longitudinally to convey cans in an upright position. The upright cans are transferred from one conveyor to the other by a vacuum transfer mechanism which is located above the adjacent ends of the conveyors. The transfer mechanism includes an endless perforated belt that travels across the open bottom of a plenum housing which is divided into a series of chambers. The chambers are subjected to a vacuum which acts to attract and hold the upright cans against the perforated belt so that the cans can be transferred by movement of the belt from one conveyor to the other, while downed cans are rejected from the conveyor system. A central chamber is subjected to a lesser vacuum than the remaining chambers, and as the cans move across this central chamber, cans with damaged upper flanges will fall from the belt to a collection site since they do not have a sufficient effective contact area to be supported by the lesser vacuum.
Another device is disclosed in U.S. Pat. No. 4,136,767 to Sarovich, which is directed to the use of a vacuum transfer apparatus to move cans from a feed-in can conveyor in an inverted position to an upper conveyor in upright position by means of a perforated endless can-carrying and can-uprighting conveyor belt which works over the peripheral surface of a rotary foraminous metal cylinder or drum. Vacuum is applied from a vacuum chamber or housing and a first vacuum control device to the perforated endless conveyor belt which lifts the cans off the feed-in can conveyor belt, whereupon the perforated endless conveyor belt grabs and holds the cans with the closed bottoms of the cans disposed against the perforated endless conveyor belt, around approximately half of the peripheral surface or circumference of an air-permeable, rotary, foraminous, metal drum or cylinder. When the cans reach the top of the drum or cylinder, the vacuum from the vacuum chamber or housing, acting through a second and upper vacuum control device and the perforated endless conveyor belt, is cut off and the cans are delivered in upright position to a take-away or delivery conveyor by which the cans may be transported to a second work station.
Although the prior art devices have been suitable for their intended purposes, they have certain shortcomings which heretofore have not been overcome. In every instance, the size of the openings through the plenum and the transfer belt is quite large so that the cubic feet per minute (CFM) of air moved through the belt is quite high. At the beginning of a cycle when no cans are on the belt, the openings in the belt are all open so that air is drawn through the belt at very high CFM, whereas the differential static pressure through the ambient air and the plenum is relatively low. As cans are picked up by the belt, an increasing number of holes become closed by the ends of the cans or containers over the belt. As this occurs, the CFM decreases and the air speed increases as the static pressure within the plenum increases. Since the pressure differential is relatively little at the beginning of the cycle, the CFM must be extremely high in order to attract the cans to the belt. Later when a large number of cans are on the belt, the static pressure differential is so great that sometimes cans which are tipped over will be drawn up against the belt rather than being separated from the other cans. Also, the greater air speed created by the much higher pressure differential will cause the air flowing through the space between adjacent cans to create a low pressure between the cans in accordance with Bernoulli's Principle. This can be undesirable where one of these cans is defective or improperly oriented. For example, the bottom of a conventional aluminum can has a chine or taper at the bottom end so that the closed bottom end has a smaller surface area than the open upper top. Thus, a vacuum transfer device can be adjusted so that only cans in the upright position will be attracted to the belt, whereas if the bottom of the can is up the surface area is too small to be held up by the vacuum and therefore is separated at the transfer station from the other cans. On the other hand, where three cans are together, one of them being upside down, if the CFM is sufficiently great, low pressure will be created in the space between the cans in accordance with the Bernoulli Principle so that the third upside down can is carried along with the other two. Thus, effective separation of the cans with the desired orientation from those which are not, is difficult and sometimes almost impossible to obtain.
Additional disadvantages with prior art devices is that the size of the fan must be very great in order to draw sufficient CFM through the transfer belt when the system starts up in order to attract a can to the belt. Thus, the power requirements for the transfer conveyor are excessive.
Additionally, it is often necessary to provide suitable venting devices or pressure regulator devices within the plenum so that the static pressure does not become so great as to collapse the ducting.
Thus, in the prior art devices, the pressure within the plenum is constantly changing, depending on the number of cans on the transfer belt, creating wide variations not only in static pressure within the plenum, but also in CFM through the belt resulting in difficult regulation and control problems.